1. Field of the Invention
The present invention relates to a unit sheath suitable to be employed in glow heaters, thermocouples and heat-exchanger tubes.
2. Description of the Prior Art
It is well known that the sheathing parts made of metals such as stainless steel or the like have been used in the envelopes for the metallic glow plugs and the sheathed-type thermocouples subjected to high temperature. The sheathing parts used in the atmosphere at more than 1000.degree. C. have been conventionally made of special heat resisting alloy such as INCONEL (trademark for a Ni--Cr--Fe alloy). The sheathing parts of ceramics have been developed for the glow plugs, or glow heaters, equipped in the compression-ignition or firing means in diesel engines, and also for the thermocouples for accurate temperature measurement of the high temperature gases and liquids. The prior thermocouples have been designed so as to be capable of making the temperature measurement in the range of from 300.degree. C. to 1400.degree. C.
On fabricating the prior glow heater disclosed, for example, in Japanese Patent Laid-Open No. 217886/1995, the outer shell of Si.sub.3 N.sub.4 is filled with a molding in which a tungsten coil is wrapped with clay material for inner shell containing Si and Ti therein. The unit shell is then burned with chemical reaction, resulting in improvement in the adherence between the outer and inner shells.
Disclosed in Japanese Patent Publication No. 19404/1985 is a heater fabricated by the steps of, embedding a heating element of metal having the high melting point, such as tungsten, molybdenum or the like, in a molding of silicon nitride, burning the silicon nitride by hot pressing simultaneously with integration of the silicon nitride with the heating element. Moreover, the prior sheathed glow plug disclosed in the above-cited No. 19404/1985, in which a coiled heating element and a resisting element, connected continuatively with each other, are arranged between the central electrode and the closed bottom of the tube made of heat resisting metal, and further the winding pitch of the resisting element is varied such that it is high at the part adjacent to the electrode and is low at the part on the side of the heating element.
In the method of fabricating the heater, it is well known that a coiled metal high in melting point covered with powdered silicon is inserted in the protective envelope of silicon nitride, and the assembly is subjected to burning with chemical reaction in nitrogen atmosphere.
It is well known, however, that tungsten or alloy thereof used in the prior heaters undergoes at the temperature of 1100.degree. C. or higher a change called recrystallization causing brittleness of metals. Any of the prior fabrication processes for the heater should require at the burning step thereof the temperature in the range of from 1400.degree. C. to 1900.degree. C., and thus the heating elements become brittlen that results in the main cause of burn-out of the heating elements. The prior fabrication processes require the expensive furnace and complicated steps with the result of the higher production cost. Because the heating elements are ordinarily subjected to the oxidation at the temperature of 600.degree. C., the prior heaters have had shortcoming such that the durability of heating elements is reduced by the oxidation due to oxygen trapped in the porous material packed in the protective envelope.